1. Field of the Invention
The present invention relates to a switching power supply device of non-linear control type.
2. Description of Related Art
FIG. 31A to FIG. 31C are each a circuit block diagram and an operation waveform view that show a conventional example of a switching power supply device employing a non-linear control system; FIG. 31A illustrates a switching power supply device that employs a hysteresis window system, FIG. 31B illustrates a switching power supply device that employs a bottom detection on-time fixing system, and FIG. 31C illustrates a switching power supply device that employs an upper detection off-time fixing system. In the meantime, any of the switching power supply devices illustrated in FIG. 31A to FIG. 31C is a voltage step-down type DC/DC converter that steps down an input voltage Vin to generate a desired output voltage Vout.
The switching power supply device of the non-linear control type has a feature that allows a high load-response characteristic to be obtained with a simple circuit structure compared with a switching power supply device of linear control type (e.g., voltage mode control type and electric current mode control type).
On the other hand, as to the switching power supply device of the non-linear control type, because of a structure that performs switching control of an output transistor by using an output ripple voltage (=a ripple component of the output voltage Vout) to drive a comparator, to correctly detect the output ripple voltage, an output ripple voltage having a somewhat large amplitude (crest value) is required. Because of this, conventionally, it is essential to use an output capacitor (e.g., electrically conductive high-molecular type) whose equivalent series resistance is relatively large, which constrains component selection and incurs cost increase.
Besides, conventionally, a technology (so-called ripple injection technology) also is proposed, in which a ripple component is forcibly injected from outside into a reference voltage Vref that is input into a comparator, whereby the comparator is driven stably. If this ripple injection technology is introduced, even if the amplitude of an output ripple voltage is not very large, it is possible to perform stable switching control; therefore, it becomes possible to use a laminated ceramic capacitor small in ESR as an output capacitor.
In the meantime, as an example of the prior art related to the above description, there is JP-A-2010-35316.
But, the amplitude (peak to peak value of the ripple component) of the reference voltage into which a ripple component is injected changes in accordance with a duty of a switch voltage Vsw (pulse voltage appearing at one terminal of an output transistor) that is used for generation of the reference voltage.
Because of this, in the conventional switching power supply device, the duty of the switch voltage Vsw changes, whereby a problem is incurred, in which a DC level of the reference voltage into which the ripple component is injected changes and line regulation deteriorates.
Besides, there is also a problem with the conventional switching power supply device that an output drop due to a circuit delay occurs especially during a high frequency operation time.